Pathologies of the peripheral nerves and muscles are common clinical conditions. Carpal Tunnel Syndrome and diabetic neuropathy affect ten million individuals in the U.S. alone. Because they are so common, disease detection and early management are often the responsibility of general practitioners, such as primary care or family physicians, occupational physicians, and endocrinologist. These conditions are, however, difficult to diagnose and evaluate using only the clinical history and physical symptoms. The only objective way to investigate peripheral nerve disease is to measure nerve conduction parameters that quantify the transmission of neural signals between two points on a nerve or between a point on a nerve and a muscle innervated by that nerve. The gold standard approach is a formal nerve conduction study by a clinical neurologist. This diagnostic test, however, has a number of important disadvantages. First, it is time consuming and requires the services of a medical expert, such as a neurologist. Second, it is costly (e.g., typically $600-$1000). Third, it is generally not available in point-of-care environments where early detection and management would be expected to have the greatest potential impact on morbidity and medical costs.
As a result of these disadvantages, formal nerve conduction studies are used in a relatively restricted fashion. They are most often employed to confirm a diagnosis prior to a significant intervention, such as surgery. The potential clinical uses of nerve conduction information, however, go well beyond these limited applications into pointof-care testing for confirmation of clinical diagnoses, as well as into intensive monitoring to detect objective evidence of response to therapeutic interventions, such as pharmacological agents and physical therapy. Peripheral nerve diagnostics that are appropriate for point-of-care environments and that are easier-to-use and less-costly are, therefore, needed.
A nerve conduction measurement typically occurs by stimulating a nerve with a short electrical impulse and then measuring the evoked response along the same nerve at a second location for a sensory or mixed nerve measurement, or over a muscle innverated by the stimulated nerve for a motor nerve measurement. Nerve conduction is most often characterized by either the latency or the conduction velocity (determined from latency and distance between stimulation and detection sites) of the evoked response. The amplitude of evoked response is also used, but to a lesser degree. The accuracy and reliability of the nerve conduction measurement is dependent on a number of technical factors. One such factor is the magnitude of the electrical stimulus used to evoke the response. In particular, the intensity of the electrical stimulus must be set within a fairly narrow range. Identifying this range is complicated by the fact that its value varies from person to person, nerve to nerve, and even from time to time in a specific nerve in an individual.
The low end of the stimulus intensity range is defined by the fact that the nerve must be stimulated at an intensity for which additional increases in stimulus intensity do not lead to further increases in the amplitude of the evoked response. This response saturation effect results from the complete activation of all the large and fast conducting myelinated nerve fibers at a stimulus intensity called the maximal stimulus intensity. Further increases in intensity may activate smaller and slower conducting fibers, but these fibers do not contribute to the early portion of the nerve response from which most traditional nerve conduction parameters are measured. Because sub-maximal stimuli do not activate the fastest conducting nerve fibers, the measured nerve conduction parameters may appear artificially slow and small, thus mimicking nerve pathology. Stimulus intensities above this maximal stimulus intensity are called supramaximal. A "near maximal" stimulus intensity (e.g., within 10-20% of maximal) is sufficient for accurate and reliable latency and conduction velocity measurements. Amplitude measurements, however, still require use of a maximal or supramaximal stimulus intensity. Although the prior art reveals a number of attempts to simplify and automate the assessment of peripheral nerve physiology, it has failed to automate and optimize the process of establishing the ideal stimulus intensity for measuring the evoked response.
The stimulus intensity is limited at the high end by a number of measurement errors and patient issues that result from overly elevated stimulus intensities. First, high stimulus intensities may lead to a "virtual cathode" effect in which the apparent point of stimulation on the nerve shifts away from the actual site of the stimulating electrode. This may lead to an artificial increase in the nerve conduction velocity or, equivalently, a decrease in the nerve conduction latency. Second, high stimulus intensities may accidentally coactivate nerves that are anatomically adjacent to the intended nerve. This can severely distort the detected signals, particularly with motor nerve measurements in which the coactivated nerve innervates muscles in the vicinity of those innervated by the intended nerve. In extreme cases, the stimulus may directly activate muscles, thereby completely bypassing the peripheral nerve and yielding incorrect nerve conduction measurement values. Third, patient discomfort is generally related to stimulus intensity. Very high stimulus intensity can cause a significant amount of discomfort and even pain.
There remains, therefore, a need for apparatus and methods for assessing peripheral nerve physiology that are easy to use, rapid and automatic. Futhermore, in order to generate accurate and reliable nerve conduction parameters, these apparatus and methods must determine the lowest possible stimulus intensity that provides a maximal or near maximal evoked response. Such apparatus and methods are needed to provide more widespread early detection, prevention, and monitoring of peripheral nerve disease. The present invention addresses these needs.